1. Field of the Invention
This invention relates generally to the field of semiconductors and specifically to a method for bonding silicon to sapphire.
2. Description of the Related Art
There are many reasons to integrate electronics in a thin film of silicon on top of an insulating substrate. Among these are increased radiation hardness, enhanced speed, higher levels of integration, and backside illumination useful for optical detectors.
Silicon On Insulator (SOI) materials have been obtained through a variety of methods. In one technique, silicon is epitaxially grown on sapphire. This technology, known as SOS, has proven useful for some applications but has been limited due to crystalline defects.
Another SOI technology is Separation by IMplanted OXygen (SIMOX). SIMOX is formed by implanting oxygen atoms into a silicon substrate. SIMOX has some of the desirable qualities of an ideal SOI, but falls short because it is time consuming to make, has a large number of impurities, and does not have a fully insulating substrate.
The Bond and Etchback Silicon On Insulator (BESOI) process is a method in which two polished silicon wafers, with an oxide layer formed on each, area placed with their polished, oxide surfaces together. The wafers are then heated to form a strong bond. One of the wafers is thinned and electronic devices are fabricated on the thinned wafer.
The BESOI technique provides very high quality silicon; however, the disadvantages of this technique are that the underlying bulk silicon wafer is coupled capacitively with the electronics on the device wafer, and that the silicon is not transparent to visible light, preventing backside illumination of electro-optical circuitry.
By using the process of bonding to adhere silicon to an insulating substrate of sapphire, a thin film of silicon on an insulator is possible, allowing devices such as high quality bipolar transistors and backside illuminated charged coupled devices (CCD) to be made.